The moon's shadow is upon us

By Douglas Furton/Special to the Tribune
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Aug 19, 2017 at 11:00 AM

About midday on a plain old Monday, Aug. 21, a darkness will sweep across the United States — a darkness so deep that birds will chirp as if night suddenly fell, a chill will settle in, and the sky will fill with stars for those in its path.

If that sort of thing sounds scary to you, then you should try to stay out of the way. But if it sounds exciting, you might start planning now to be in the right place at just the right time.

On Monday, the moon will pass exactly between the sun and Earth, producing what we call a solar eclipse. During a solar eclipse, the moon completely blocks out the sun, turning day into night and revealing to an observer in the right place the sun's tenuous, wispy outer atmosphere, which is normally lost in the sun's daytime glare.

Solar eclipses are not all that uncommon. Each year there are at least four solar eclipses; some years, there are even more.

At first blush, we might expect as many as 12 solar eclipses each year: one each month, when the moon, in its orbit around Earth, slips between Earth and the sun. But the moon's orbit is slightly inclined (about 5 degrees) to the plane of Earth's orbit around the sun, causing the moon to more often than not drift unspectacularly above or below the sun from our Earthbound perspective.

Yet still one might wonder: If there are at least four total solar eclipses every year, why haven't I seen one? Ask around — you'll find that very few people have ever seen a total solar eclipse. Surely such a spectacle cannot go unnoticed.

To understand why total solar eclipses are so rare, it is helpful to consider the situation from a different perspective. The moon casts a shadow, as does every opaque body that is illuminated by the sun. The central part of any shadow — called the umbra — is very dark. Surrounding the umbra of a shadow, however, is a darkened (but not completely dark) region called the penumbra.

The moon's umbral shadow is shaped like a slender tapering cone that points away from the sun. By happenstance, the sizes of and distances to the sun and moon conspire to make it so the moon's umbral shadow just reaches Earth. So perfectly balanced are size and distance that on Earth's surface the diameter of the moon's dark umbra is only about 60 miles.

Viewed from this perspective, it becomes apparent why total solar eclipses are so rare. In order to see a total solar eclipse, you have to be in just the right place precisely when the tiny tip of the moon's umbral shadow sweeps over you.

During Monday’s solar eclipse, the moon's shadow will sweep from northwest to southeast diagonally across the full extent of the continental United States. The shadow will make landfall very nearly at the start of the eclipse near a town called Lincoln City, Oregon, just west of Salem, at about 1:20 p.m. EDT, and later slips off to sea near Charleston, South Carolina, at about 3:50 p.m.

The 60-mile-wide blotch of darkness covers the almost 2,600-mile distance from Oregon to South Carolina in an hour and a half, moving at a speed of about 1,700 mph. For comparison, that is about equal to the top speed of an F-16 jet.

The area across the U.S. swept out by the moon's shadow during the eclipse — 60 miles wide by 2,500 miles long — is about 150,000 square miles. The total surface area of Earth is about 200 million square miles. So, with a little bit of area added to each end of the eclipse path, it only covers about 0.1 percent of Earth's surface.

That's why solar eclipses are so rare: You have to be in just the right place at just the right time to see one.

If you don't want to travel into the path of totality during Monday’s eclipse, the day here in Michigan will still be very unusual. Observers here will see a partial solar eclipse; the moon will only partially block the sun. But the Great Lakes region is deep within the moon's penumbral shadow, and at mid-eclipse, about 2:30 p.m., the moon will block a little more than 80 percent of the area of the sun's surface.

WHERE TO WATCH

If you’re interested in watching Monday’s eclipse, the Shoreline Amateur Astronomical Association will be holding a program at Hemlock Crossing Park, 8115 West Olive Road, West Olive, from 1-4 p.m.

— Douglas Furton is a professor of physics at Grand Valley State University.